(i) Field of the Invention
The present invention relates to an optical information recording medium, a composition for an optical information recording film, and a method for preparing the optical information recording medium. More specifically, it relates to a writing once optical information recording medium having compatibility with a compact disc, a composition for its recording film, and a method for preparing the recording film.
(ii) Description of the Prior Art
Some of optical information recording media using a laser beam have already been put to practical use as large capacity data memories.
In particular, compact discs (CDs) and CD-ROMs have been utilized as rapid access digital recording media having large capacity to memorize voices, images, code data and the like, and they have spread in markets. However, all of them are read only memories, and so in these media, anything cannot be recorded. Hence, a write-once optical recording medium is desired in which recording/editing can be optionally done by a user and which has compatibility with CDs and CD-ROM players largely spread in the markets.
Thus, there have been suggested and developed the optical recording media, i.e., CD-R media in which the recording can be carried out in accordance with compact disc (CD) standards [Nikkei Electronics, No. 465, p. 107, Jan. 23, 1989; and Optical Data Storage Technical Digest Series, Vol. 1, p. 45 (1989)]. The CD-R medium is formed by laminating a recording layer, a reflective layer and a protective layer in this order on a transparent resin substrate. When the recording layer is irradiated with a laser, a pit is formed in the recording layer, and the detection of a signal is carried out by a reflectance change at this site. This medium has a single plate structure having a thickness of 1.2 mm so as to satisfy CD standards, and the pits having 9 kinds of length at an interval of T in the range of from the shortest 3T pit length to the longest 11T pit length (T=231.4 ns) are used in accordance with a modulation method of a CD system, i.e., an EFM (eight to fourteen modulation) system. Therefore, in the CD-R medium, the pits corresponding to the 9 kinds of predetermined length are formed by the laser irradiation, and the pit length is reproduced by the detection of the thus formed pit edges.
In the recording system of the write-once optical information recording medium, particularly the CD-R medium, heat mode recording (thermal recording) which has undergone light/heat conversion is usually employed as a practical level. Therefore, as the composition for the recording film, there have been suggested low-melting metals, organic polymers and some organic dyestuffs which give rise to a physical change or a chemical change such as melting, vaporization, sublimation or decomposition. Above all, the organic dyestuffs which have a low thermal conductivity, a low melting point or a low decomposition temperature are preferable from the viewpoint of recording sensitivity. In addition, these organic dyestuffs are also preferable in point of optical design, because they can hold high reflectance for CD compatibility. In consequence, much attention has been mainly paid to cyanine dyestuffs, metal phthalocyanine dyestuffs, naphthoquinone and azo dyestuffs, and the recording layers have been developed from these dyestuffs.
Heretofore, some examples have been disclosed in which the CD-R media are constituted of the organic dyestuffs.
Hamada et al. have suggested and disclosed a CD-R medium in which the optical recording layer comprises a layer containing a cyanine dyestuff in Japanese Patent Application Laid-open No. 147286/1990. The medium system has high reflectance and good recording sensitivity.
However, the present inventors have found that this suggested invention has some problems. That is, since the recording layer comprises the cyanine dyestuff, error rate and jitter properties deteriorate noticeably under a high-temperature and high-humidity environment, and light resistance also declines, and so when data communication is often carried out with a CD, the reliability of the medium for a long time is poor. As more serious troubles, in EFM pit length recording, the formation stability of particularly the 3T pit edge is not always good, and problems of the jitter properties and the error rate take place at times.
Japanese Patent Application Laid-open No. 215466/1991 discloses a CD-R medium in which the optical recording layer comprises a phthalocyanine dyestuff having a specific substituent. The recording film comprising this dyestuff is excellent in light resistance, humidity resistance and heat resistance, so that the optical recording medium having good balance between reflectance and recording sensitivity can be provided. In this dyestuff system, however, we have elucidated the feature that in the EFM pit length recording, particularly the pit of 3pit length is formed more largely than in the prepits of the commercial CD and CD-ROM, and we have found that defective playback on the commercial or conventional CD player cannot be completely avoided owing to this feature of the pit edge.
On the other hand, with regard to the optical information recording media, some examples have been disclosed in which a certain kind of additive is added to the recording layer to improve the characteristics of this layer.
In Japanese Patent Application Laid-open No. 86787/1980, the improvement of recording sensitivity is intended, and a light absorber for a recording laser beam is added to the recording layer comprising an organic dyestuff or a resin to improve a light/heat conversion efficiency, whereby permitting the formation of the recording pits even by the laser irradiation of lower power. This light absorber itself does not have any influence on the recording threshold performances of the organic dyestuff or the resin having a recording function, for example, the thermal decomposition temperature of the organic dyestuff, and thus, substantially, the stability of the recording pits themselves formed by the heat mode recording is not always good. Additionally, in optical information recording media such as the CD-R medium in which the retention of the high reflectance for the CD compatibility is required, the deterioration of the reflectance which is inevitably caused by the addition of the light absorbent is not preferable.
Furthermore, Japanese Patent Application Laid-open Nos. 16888/1983, 62839/1983 and 92448/1984 disclose examples in which the recording sensitivity is mainly improved by adding an additive having self-oxidizing properties, for example, a nitro-based compound (such as nitrocellulose) to the organic dyestuff layer which is the recording layer. It has been confirmed that in this nitro-based system, the heat generated at the exothermic self-oxidizing decomposition of the additive functions as an effective heat source on recording, and the influence of the organic dyestuff itself having the recording function on the recording threshold properties and decomposition properties is observed sometimes. However, in this nitro-based system, the formation of the pits inevitably involves the rapid heat generation at the time of the oxidizing decomposition, and so the uniformity of the formed recording pit-edges is not always good. From experiments in which nitrocellulose is added, the present inventors have observed that particularly in the case that the pit length recording for the CD compatibility is carried out in the EFM system, the noticeable deterioration of signal qualities such as jitter properties is not avoidable.
In Japanese Patent Application Laid-open No. 23-9943/1986, Nanba et al. disclose the improvement of the light resistance of an optical recording layer by the use of the mixture system of an indolenine cyanine dyestuff and a dithiole transition metal complex, and additionally, in Japanese Patent Application Laid-open No. 25493/1992, they also describe the application of a mixture film comprising a cyanine dyestuff and the above-mentioned dithiole transition metal complex in a CD-R medium. In this system, the durability, particularly the light deterioration of the recording layer which is considered a problem can be improved to some extent without impairing the recording properties of the cyanine dyestuff. However, in this technique, it has been confirmed that the dithiole metal complex functions, for the improvement of the durability of the medium, as an effective quencher for singlet oxygen which is considered to be the main cause of the light deterioration of the cyanine dyestuff, but the effect and improvement of characteristics in the pit length recording and the pit-edge detection as neither recognized nor referred to.
Other examples have been present in which the characteristics of the optical information recording medium are improved by an intramolecular or an intermolecular function of the organic dyestuff and a certain metal compound, but most of them are mainly concerned with the improvement of the durability of the above-mentioned recording film or the matching of the recording film with a recording laser wavelength in an absorptive section. Therefore, the effect of the used organic dyestuff on the recording threshold properties and the like is neither expected nor referred to.
As stated above, in the optical information recording media which have been heretofore developed, the control of the pit-edge is not always sufficient, and for this reason, the application of the media to the pit length recording is particularly difficult. Specifically, in the CD-R medium which is one typical example of the conventional media, the compatibility with the CD of the read only memory is not always sufficient, and in the playback on the prevalent commercial CD player, the CD-R medium gives rise to a problem at times.
The present inventors have analyzed and investigated this problem, and as a result, they have found that the jitter components increase owing to the poor control of the pit-edge formed at the time of the laser beam irradiation, and that particularly in the pit length recording, the recording pits/lands are formed, deviating from a predetermined pit length or a pit interval length (hereinafter referred to as “land length”). This means the deviation from the pit standard length, and so it will be referred to “deviation properties.” Thus, it has been found that in the process of data playback from the recording pits, the read error of a signal length tends to occur, and property deterioration such as the increase of the error rate takes place. In addition, it has been also found that this deterioration behavior is very liable to occur at the time of the formation of the pit length/land length which is smaller than the diameter of the irradiated laser beam.
Here, selecting the CD-R medium as an example, the shortest 3T pit length (0.83-0.97 micron) and the shortest 3T land length are required to be stably formed, avoiding light interference and heat interference in the recording film, in accordance with the CD recording system (EFM system) by the irradiation of laser beam having an effective radius of about 1 micron (usually by the use of a semiconductor LD in a near infrared region). However, in the conventional recording film design, this pit-edge control is very difficult. Therefore, particularly the formed 3T pits are excessively larger or smaller than the 3T prepit length of the commercial CD (in which compatibility with the CD player is sufficiently secured), and the deviation properties are poor and the jitter value is also high, so that errors are often made. In the worst case, it has been found that the problem of the defective playback takes place in the CD player.